The invention is directed toward the field of optical communications, more particularly to wireless optical communications, and even more particularly to wavelength division multiplexed wireless optical communication.
Some optical-signal-based communication systems are wireless, i.e., the medium in which the signal propagates is free space. In contrast to radio frequency (RF) communication, optical wireless has a disadvantage, in some respects, of being extremely directional. This requires very precise alignment between the transmitting telescope and the receiving telescope. An advantage of the extremely directional nature of wireless optical communication is that it is secure. To intercept the signal, it is necessary to be on the path of the transmitted light.
Another advantage of the optical wireless link is that the optical portion of the spectrum is not a form of communication regulated by the government. In other words, no license is needed to operate the transmitter and receiver. In contrast, such a license would be necessary for a comparable radio frequency (RF) wireless communication system.
Some optical-signal-based communication systems are limited to the use of a transmission medium of optical waveguides. Wireless optical communication has an advantage over optical fiber-based communication in that the wireless communication does not require a physical connection between the source of the signal and the device that receives the signal. In a setting such as an urban area like New York City, it can be very difficult to install a physical connection between buildings, especially if a street and/or one or more other buildings separate the buildings. A wireless optical link only requires an unobstructed path between the transmitter and the receiver. In the New York City situation, this is much easier to achieve than the installation of a physical link. The lack of a physical connection can also be advantageous where temporary high capacity data links between computing installations are required, such as in an emergency relief operation for a disaster area or in military operations.
A known wireless optical link includes a transmitting telescope, for forming a transmitted beam, aimed at a second telescope that collects the received beam. The medium in which the beam propagates is the atmosphere. Typically, the optical signal to be transmitted is emitted from a semiconductor laser. The emitting facet of the laser lies at the front focal plane of the transmitting telescope. Conversely, the received signal is typically collected on a photodetector that lies at the back focal plane of the receiving telescope.
The prior art transmitter uses only a single wavelength of light. Due to the great difficulty of implementing single wavelength operation, multiple wavelength operation did not develop.
The prior art transmitting telescope and the receiving telescope are precisely aimed at one another, again, because optical signals are extremely directional. Atmospheric diffraction effects can cause the transmitted beam to vary in intensity (scintillation) and to deviate from the carefully aimed path (beam wander). To compensate for this problem, multiple element (i.e., multiple apertures) transmitting telescopes have been used. The multiple apertures represent redundant sources of the optical signal. Although each of the signals from a multiple aperture telescope may be attenuated by optical diffraction effects, the multiple attenuated signals represent an equivalent signal strength to a non-attenuated single signal.
Multiple-aperture receiving telescopes are also used in the prior art wireless optical communication system. This provides a greater optical signal collection area. The signal collected by each receiving element (or aperture) is sent down a respective optical fiber and an Nx1 optical coupler is used to flannel the collected optical signals from these fibers into a single output fiber. Conversely, each transmitting element or aperture in the multiple aperture transmitting telescope is supplied with an optical signal by a respective emitter coupled to a respective optical fiber.
The prior art utilized multiple mode (multimode) optical sources due to their much greater availability and much lower cost. Consequently, the prior art transmitter is configured for multimode light propagation within its optical structures. As a result, the prior art optical wireless link has limited power, resulting in limited transmission distances. Also, the prior art optical wireless link is limited in bandwidth, which for the single wavelengths has attained a maximum of about 2.5 giga bits per second (Gbits/sec).
Despite the desirability of the wireless optical link, the prior art optical wireless technique is still not satisfactory because it is limited in bandwidth. More importantly, though, the prior art wireless optical link is not powerful enough to permit a useful transmission distance. For example, the prior art wireless optical link is not powerful enough to overcome bad weather, such as rain, fog or snow. Also, the prior art optical wireless link cannot overcome the problems of scintillation.
It is an advantage of the invention that the problems of the prior art, in particular insufficient power and insufficient bandwidth, are overcome. In overcoming these and other problems, the invention (among other things) provides techniques for multiplexing and demultiplexing a free space, bi-directional laser communication data link for single channel and wavelength division multiplexing (WDM) applications.
The invention is, in part, a recognition that the output power of the transmitter can be increased by using single mode optical structures between the lasers and the focal plane of the transmitting telescope, regardless of whether the wireless medium is a multiple mode (multimode) medium. Such single mode optical structures preferably include one or more single mode optical amplifiers to provide the necessary gain to the optical signal. Also, to provide the needed increase in bandwidth, the invention sends information over multiple wavelengths, rather than the single wavelength of the prior art optical wireless technique.
The present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.